Re: [PATCH v3 01/10] KVM: MMU: fix decoding cache type from MTRR

Paolo Bonzini
2015-07-13T15:13:08Z

On 13/07/2015 16:45, Xiao Guangrong wrote:
>> + /* MTRR is completely disabled, use UC for all of physical
>> memory. */
>> + if (!(mtrr_state->enabled & 0x2))
>> + return MTRR_TYPE_UNCACHABLE;
>>
>> actually disappears in commit fa61213746a7 (KVM: MTRR: simplify
>> kvm_mtrr_get_guest_memory_type, 2015-06-15).
>
> :(
>
> Based on the SDM, UC is applied to all memory rather than default-type
> if MTRR is disabled.

There are two issues I think. One is that I cannot find in the current
code that "UC is applied to all memory rather than default-type if MTRR
is disabled". mtrr_default_type unconditionally looks at
mtrr_state->deftype.

> However, fast boot came back if "return 0xFF" here. So fast boot expects
> that the memory type is WB.

Yes.

>>
>> static u8 mtrr_default_type(struct kvm_mtrr *mtrr_state)
>> {
>> if (mtrr_is_enabled(mtrr_state))
>> return mtrr_state->deftype &
>> IA32_MTRR_DEF_TYPE_TYPE_MASK;
>> else
>> return MTRR_TYPE_UNCACHABLE;
>> }
>>
>> ? Then it's easy to add a quirk that makes the default WRITEBACK until
>> MTRRs are enabled.
>
> It is the wrong configure in OVMF... shall we need to adjust KVM to satisfy
> OVMF?

That's what quirks are for... The firmware should still be fixed of course.

Paolo